Oilpan: improve ScrollableArea handling.

Source/core/frame/RootFrameViewport.cpp

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 #include "core/frame/RootFrameViewport.h"
 
 #include "core/frame/FrameView.h"
 #include "core/layout/ScrollAlignment.h"
 #include "platform/geometry/DoubleRect.h"
@@ skipping 310 matching lines @@
     ScrollableArea::serviceScrollAnimations(monotonicTime);
     layoutViewport().serviceScrollAnimations(monotonicTime);
 }
 
 void RootFrameViewport::updateCompositorScrollAnimations()
 {
     ScrollableArea::updateCompositorScrollAnimations();
     layoutViewport().updateCompositorScrollAnimations();
 }
 
+DEFINE_TRACE(RootFrameViewport)
+{
+    visitor->trace(m_visualViewport);
+    visitor->trace(m_layoutViewport);
+    ScrollableArea::trace(visitor);
+}
 
 } // namespace blink